A New Pyrene-Based Fluorescent Probe for the Determination of

J. Phys. Chem. B 2007, 111, 12985-12992

12985

A New Pyrene-Based Fluorescent Probe for the Determination of Critical Micelle Concentrations Andreas Mohr,‡,† Peter Talbiersky,‡ Hans-Gert Korth,‡ Reiner Sustmann,*,‡ Roland Boese,§ Dieter Bla1 ser,§ and Heinz Rehage# Institut fu¨r Organische Chemie, UniVersita¨t Duisburg-Essen, Campus Essen, UniVersita¨tsstrasse 5, D-45117 Essen, Germany, Institut fu¨r Anorganische Chemie, UniVersita¨t Duisburg-Essen, Campus Essen, D-45117 Essen, Germany, and Lehrstuhl fu¨r Physikalische Chemie II, UniVersita¨t Dortmund, D-44221 Dortmund, Germany ReceiVed: April 24, 2007; In Final Form: August 7, 2007

A new pyrene-based fluorescent probe for the determination of critical micelle concentrations (CMC) is described. The title compound 1 is obtained in five steps, starting from pyrene. Fluorescence spectroscopic properties of 1 are studied in homogeneous organic solvents and aqueous micellar solutions. In a wide range of organic solvents, probe 1 exhibits a characteristic monomer emission of the pyrene fluorophore, with three distinct peak maxima at 382, 404, and 425 nm. The spectra change dramatically in aqueous solution, where no monomer emission of the pyrene fluorophore is detected. Instead, only strong excimer fluorescence with a broad, red-shifted emission band at λmax ) 465 nm is observed. In micellar aqueous solution, a superposition of the monomer and excimer emission is found. The appearance of the monomer emission in micellar solution can be explained on the basis of solubilization of 1 by the surfactant micelles. The ratio of the monomer to excimer fluorescence intensities of 1 is highly sensitive to changes in surfactant concentration. This renders 1 a versatile and sensitive probe molecule for studying the micellization of ionic and nonionic surfactants. For a representative selection of common surfactants, the critical micelle concentrations in aqueous solution are determined, showing excellent agreement with established literature data.

Introduction Surfactant molecules and micellar solutions are of fundamental importance in a wide variety of fields, ranging from material science to catalysis and controlled drug delivery.1-3 The most important parameter for the characterization of surfactant solutions is the critical micelle concentration (CMC).4-6 Various techniques are routinely used to determine the CMC in aqueous solution. The most commonly applied methods are conductivity, voltammetry, calorimetry, scattering techniques, surface tension, UV/vis, and fluorescence spectroscopy, which all are based on an abrupt change in the related physical properties upon micelle formation.7 Particularly, luminescence probing techniques have advanced rapidly over the past three decades, as a result of the development of a large number of dyes and specific probe molecules.8-13 Since fluorescence spectroscopy is quite more sensitive than optical absorption, the design of new fluorescent probe molecules is a subject of intense research.14-19 A frequently used fluorescence probe is pyrene, whose fluorescence maxima at λmax ) 373 and 383 nm, labeled peaks I and III, are sensitive to the local environment. From the change of the intensity ratio of I/III as a function of surfactant concentration, the CMCs of numerous surfactants have been determined.9,20 * To whom correspondence should be addressed. E-mail: reiner. [email protected]. Phone: +49-201-183-3097. Fax: +49201-183-4259. ‡ Institut fu ¨ r Organische Chemie, Universita¨t Duisburg-Essen. § Institut fu ¨ r Anorganische Chemie, Universita¨t Duisburg-Essen. # Universita ¨ t Dortmund. †Present address: Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, S-221 00 Lund, Sweden.

Fluorescent nitric oxide cheletropic traps (FNOCTs) are quinodimethane-type molecules that specifically trap nitric oxide (NO) with high sensitivity.21,22 FNOCTs with a fluorescent phenanthrene system have previously been synthesized, and their physical and chemical properties have been studied in detail.23 Due to the sensitivity with which nitric oxide is trapped, FNOCTs were advantageously applied in cell biological studies to quantitatively determine NO production.23,24 As a part of our continuing interest in the synthesis of fluorescent pyrene-based FNOCTs, we have become particularly attentive to the fluorescence spectroscopic properties of the polycyclic ketone 1, a FNOCT precursor. We have found that for a wide range of organic solvents, the emission behavior of compound 1 is independent of the solvent properties, showing a characteristic monomer emission of the pyrene fluorophore in the 370-400 nm range. However, to our surprise, the emission spectrum changed dramatically in aqueous solution. When fluorescence spectra of 1 were measured in water, the characteristic monomer emission of the pyrene fluorophore could no longer be detected. Instead, we found that in the 1-100 micromolar concentration range, compound 1 showed an excimer fluorescence only. In mixtures of water-miscible organic solvents and water, both the monomer and the excimer emission of 1 were recorded. The same was found upon addition of surfactants to aqueous solutions of 1. This fact suggested that 1 may be advantageously used as a probe for studying the micellization of surfactants in aqueous solutions. We here report on the synthesis of 1 and its fluorescence emission properties in aqueous solution in the absence and in the presence of various surfactants. For a representative selection of ionic and nonionic surfactants, it is

10.1021/jp0731497 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/24/2007

12986 J. Phys. Chem. B, Vol. 111, No. 45, 2007 demonstrated that 1 is a versatile and very sensitive probe for the reliable determination of critical micellar concentrations.

Experimental Section Materials. The procedures for preparation of compound 1, analytical and spectroscopic data, and related instrumentation are provided as Supporting Information. Sodium dodecyl sulfate (SDS), hexadecyltrimethylammonium bromide (CTAB), n-dodecyl-β-maltopyranoside, and polyoxyethylene(20) sorbitan monolaurate (Tween 20) were purchased from Fluka. Polyoxyethylene(10) isooctylphenyl ether (Triton X-100) was from Aldrich. Polyoxyethylene(20) cetyl ether (Brij-58) was purchased from Acros Organics. Tetraethylene glycol monododecyl ether (C12E4) and octaethylene glycol monododecyl ether (C12E8) were obtained from Nikko Chemical Co. All surfactants were used without further purification. Water was doubly distilled, and all measurements were carried out with freshly prepared solutions at 22 ( 1 °C. X-ray Diffraction Measurements. Experimental details and diffraction data are given in the Supporting Information. Crystallographic data for structure 1 have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-644609. Copies of the data can be obtained free of charge upon application on CCDC, 12 Union Road,CambridgeCB21EZ,U.K.(E-mail: [email protected]). Determination of the Critical Micelle Concentration of the Surfactants in Aqueous Solution. In the present study, 1 was used as a probe to determine the CMC of SDS, CTAB, Triton X-100, n-dodecyl-β-maltopyranoside, Brij-58, Tween 20, C12E4, and C12E8 in aqueous solution at 22 ( 1 °C. Fluorescence emission spectra were recorded on a JobinYvon JY3 spectrofluorimeter equipped with a thermostatted cell holder, a Hamamatsu R212 UH photomultiplier, and a slit system set. Fluorescence of 1 was recorded from 360 to 650 nm after excitation at 345 nm (half-bandwidth ) 4 nm). In micellar solutions, the emission spectrum of 1 showed three significant peaks at 382, 404, and 425 nm, respectively. The CMC was determined by measuring the intensity of the first peak at 382 nm as a function of surfactant concentration. Plots of the fluorescence intensity versus surfactant concentration were properly fitted by a sigmoid function of the Boltzmann type of the form y ) (A1 - A2)/{1 + exp[-(x - x0)/∆x]} + A2,20 where y corresponds to the fluorescence intensity at 382 nm. The variable x is the total surfactant concentration, x0 is the center of the sigmoid, ∆x is the slope factor, A1 is the upper limit of the fluorescence intensity, and A2 is the background fluorescence at zero surfactant concentration. The critical micelle concentrations were then derived from the inflection points of the fitted sigmoid traces. Results and Discussion Synthesis. The preparation of fluorescence probe 1 comprises five steps, as summarized in Scheme 1. Pyrene-4,5-dione (2) was obtained by catalytic oxidation of pyrene with ruthenium(III)chloride hydrate (RuCl3‚H2O) and sodiumperiodate (NaIO4) under mild conditions.25 In the next step, 2 was subjected to an

Mohr et al. aldol condensation with 3-pentanone in order to give the desired 2′,5′-dimethylpyrene-4,5-cyclopenta-2′,4′-diene-1′-one (3).26 However, all attempts to get 3 by this means were without success. Instead, the base-catalyzed condensation of 2 with 3-pentanone always gave 2′,5′-dimethyl-4′-hydroxypyrene-4,5-cyclopenta2′-en-1′-one (4) in good yield (74%). Therefore, 4 was reacted with acetylchloride to give 2′,5′-dimethyl-2′-chloropyrene-4,5cyclopenta-1′-one (5) as a 4:1 mixture of two stereoisomers in 91% yield. The following base-induced dehydrochlorination in the presence of potassium hydroxide gave 6, the Diels-Alder dimer of 3, in a yield of 50%. This course of reaction is similar to the one described previously by Jones27a in a study of DielsAlder additions of 1,3-dimethylcyclopenta[l]phenanthrene-2-one. Similar to what has been reported for the dimer of this compound in the 1H NMR spectrum of dimer 6, only two signals for the four methyl groups were observed.27b The pairwise equivalency of two methyl groups is due to a fast degenerate Cope rearrangement. Because the detailed analysis of this interesting reaction is beyond the scope of this study, it is provided as Supporting Information for the interested reader. In the last step, dimer 6 was heated to 120 °C in the presence of a tenfold excess of norborn-5-ene-2,3-(E)-dicarboxylic acid diethylester (7) as a dienophile. At this temperature, the retroDiels-Alder reaction of 6 in-situ produced 3, which was further trapped by 7 to give 3′,5′-dimethyl-3′,5′-(4,5-pyrenylene)tricyclo-[5.2.1.02′,6′]-decan-4′-on-8′-exo-9′-endo-dicarboxylic acid diethylester (1). The course of the reaction was followed by thin-layer chromatography. After 7 h of reaction time, excess 7 was removed by vacuum distillation at 150-160 °C. The oily residue was purified by chromatography on silica gel (dichloromethane/n-hexane, 4:1) to give ketone 1 in 43% yield. All products were characterized by IR, NMR, and mass spectrometry and, in part, by elemental analysis. Compound 1 was additionally characterized by homonuclear nuclear Overhauser effect correlated 2D NMR (NOESY) spectrometry and X-ray structural analysis. X-ray Molecular Structure of 1. The structure of 1 was elucidated by twinned crystal X-ray diffraction. The X-ray analysis reveals that 1 possesses an anti/exo-structure with respect to the orientation of the carbonyl group versus the norbonene methylene bridge (Figure 1). Fluorescence Emission Behavior of 1 in Homogeneous Solution. The fluorescence emission behavior of 1 was studied in a range (22) of polar and nonpolar solvents. Figure 2 shows some typical fluorescence emission spectra of 1 in selected nonaqueous solvents and water. In all organic solvents, the characteristic monomer emission of the pyrene fluorophore with three distinct peak maxima at 382 (peak I), 404 (peak II), and 425 nm (peak III) was detected. The fluorescence intensity was found to increase in going from nonpolar to polar solvents. In DMSO, the fluorescence intensity of 1 is roughly three times higher than that in n-hexane. However, no reasonable correlation with typical parameters of solvent polarity, such as the dielectric constant  or ET(30), was found.28-30 To examine whether there is a specific solute-solvent relationship, we have also focused on the relative peak intensities of the well-defined emission bands I, II, and III. With respect to I, the relative peak intensities of II and III were almost the same [1.00:(0.71 ( 0.03):(0.27 ( 0.02)], indicating that the emission behavior of 1 is not noticeably influenced by specific solute-solvent interactions (see Supporting Information, Table S6). Furthermore, evident from the spectra of Figure 2, no excimer fluorescence was observed for the entire range of organic solvents studied.

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SCHEME 1: Synthesis of Fluorescence Probe 1a

a Reagents and conditions: (i) CH Cl , MeCN, H O, RuCl ‚H O, NaIO , 23 °C, 24 h; (ii) EtOH, KOH, 23 °C, 24 h; (iii) toluene, CH COCl, 2 2 2 3 2 4 3 0 °C, 1 h, then 23 °C, 1 h; (iv) CH2Cl2, EtOH, KOH, 0 °C, then 23 °C, 30 min, 10% HCl; (v) 120 °C, 7 h.

Figure 1. X-ray molecular structure of 1.

In sharp contrast to the fluorescence properties in nonaqueous solution, a broad, red-shifted band at around 400-550 nm (λmax ) 465 ( 3 nm) was observed in aqueous solution (Figure 3). This emission was found to be prominent even at very low concentrations (g0.3 µM) (Figure 3b). The peak intensity of

this band increased linearly with increasing concentration of 1 up to 15 µM. At higher concentrations of 1, the fluorescence increase was reduced due to the limited solubility of 1 in water (Figure 3a, inset). The foregoing observations clearly indicate the formation of excimers, in accord with the fact that excimer

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Figure 2. Fluorescence emission spectra (λexc ) 345 nm) of 1 (10 µM) in selected solvents at 22 ( 1 °C. Maxima at 382, 404, and 425 nm were assigned as peaks I, II, and III, respectively.

formation and fluorescence are common to most aromatic hydrocarbons and their derivatives.31,32 Remarkably, 1 does not exhibit any characteristic monomer emission of the pyrene fluorophore at concentrations as low as 0.3 µM. Thus, no ratio of excimer to monomer emission could be determined in water. We therefore hypothesized that 1 may be used as a sensitive fluorescent probe to study the micellization of surfactants in aqueous solution. It appeared conceivable that above the critical micelle concentration (CMC) of the surfactant, compound 1 might be incorporated into the micellar aggregates, thereby giving rise to monomer emission of the pyrene fluorophore and thus indicating the formation of micellar assemblies. Therefore, the fluorescence spectroscopic properties of 1 in aqueous solutions of common surfactants were conducted. Pyridinium N-phenol betaine, called ET(30), has been used as a dye molecule to probe the polarity, or rather the effective dielectric constant f of its environment at the aqueous interfacial region of micelles, microemulsions, and phospholipid bilayers.33 As shown for a number of ionic and nonionic surfactants, ET(30) is located within the micelle-water interface, thus sensing changes in the micropolarity caused by salt addition and variation of surfactant chain length, concentration, counterion, and temperature.33 However, apart from the hydrophilic surfactant head groups and counterions, only water and alkyl chains contribute to the polarity of the aqueous interface. Therefore, mixtures of alcohol/water or dioxane/water have been taken as model systems to mimic the polarity of the interfacial region. Since ET(30) values have been determined for a large number of solvents and solvent mixtures, data for the interfacial polarity of the micelles could be translated into effective dielectric constants.29,30,33 ET(30) values are simply defined as the molar electronic transition energies (in kcal mol-1) of dissolved indicator dye at 25 °C. High ET(30) values correspond to high solvent polarity. Since the polarity of the abovementioned mixtures decreases with increasing fraction of organic cosolvent, the fluorescence emission behavior of 1 was also investigated in mixtures of dioxane/water, ethanol/water, and methanol/water. In fact, already in mixtures containing small mole fractions of organic solvents, the monomer emission of 1 at 382, 404, and 425 nm could be detected in addition to the excimer emission. As mentioned earlier, no reasonable correlation with the empirical parameter ET(30) was found. Plots of the monomer fluorescence intensity at 382 nm as a function of the molar fractions are displayed in Figure 4a, showing a

Mohr et al. sigmoidal shape. The data reveal that excimer formation is more facilitated the more “water-like” the solvent is. This suggested that changes in the fluorescence intensity might be exclusively due to changes in the effective dielectric constant. This assumption was nicely confirmed when the data of Figure 4a were plotted as a function of the dielectric constant (Figure 4b). Thus, from the “master curve” of Figure 4b, it can be inferred that an increase in the fluorescence intensity in mixtures of water/organic solvent directly correlates with a decrease in the effective dielectric constant. Fluorescence Emission Behavior of 1 in Surfactant Solution. A micelle-water interface is different from the aqueous bulk phase in many properties, and the effect of surfactant micelles on reaction rates and acid-base equilibria is wellknown.37-40 Acid-base properties of a number of indicator dyes have been studied extensively in aqueous micellar solution. Dye molecules bound to micelles of ionic and nonionic surfactants show significant shifts in the apparent acidity constants, which can be attributed to the above-mentioned factors, such as electrostatic potential at the micelle surface, ion exchange between the micelle-water interface and the aqueous bulk phase, low interfacial effective dielectric constants, and interfacial salt effects.38,41-45 As mentioned earlier, the micelle-enhanced solubilization of hydrophobic drugs is of major practical importance.1-3 Much attention has been given to both the extent of solubilization and the possible positions at which solubilization processes occur. Electrostatic and hydrophobic interactions are the main driving forces for micellar aggregation. Due to the interplay of hydrophilicity and hydrophobicity, probe molecules may thus be solubilized at a number of different sites. In aqueous systems, hydrophilic molecules are dissolved in the bulk water phase or slightly adsorbed at the micelle-water interface. As the solubility in water decreases, these molecules can be solubilized either between the hydrophilic head groups or in the palisade layer of the micelle between the hydrophilic head groups and the first few carbon atoms of the hydrophobic tail. Finally, substances that are sparingly soluble in water are preferably located in the core of the micelle. Our conclusion that 1 may be used as a sensitive probe molecule was confirmed by fluorescence spectroscopic studies in surfactant solutions. The emission characteristics of 1 were first measured at room temperature in solutions of two ionic surfactants. Figure 5 presents typical fluorescence spectra of 1 in aqueous solutions of SDS and CTAB, at concentrations below and above the critical micelle concentration. In the presence of a low concentration of surfactant (5.0 mM SDS and 0.6 mM CTAB), a broad redshifted band was observed at 400-550 nm. As mentioned earlier, this emission can be ascribed to the excimer formation of 1 in water in the presence of a low concentration of surfactant. As the surfactant concentration was increased to 7.0 mM SDS and 0.9 mM CTAB, respectively, the fluorescence spectrum of 1 changed dramatically, showing both the characteristic monomer emission of the pyrene fluorophore and the broad, featureless emission band at 465 nm. Excimer and monomer both exist near the critical micelle concentration, and the monomer obviously becomes more dominant as the surfactant concentration is further increased. Finally, when the surfactant concentration was far above the CMC (10.0 mM SDS and 1.20 mM CTAB), only the well-defined emission spectra of the monomeric form of 1 was observed, indicating that almost all probe molecules exist as monomers incorporated into the hydrophobic region of the micelles. Therefore, our results imply that the

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Figure 3. Fluorescence emission spectra (λexc ) 345 nm) of 1 in water. (a) [1] ) 0-40 µM, and (b) [1] ) 0-1.7 µM. Insets: plot of the fluorescence intensity at λem ) 465 nm versus [1].

Figure 4. (a) Fluorescence intensity of 1 (10 µM) at λem ) 382 nm (monomer emission peak I, λexc ) 345 nm) in mixtures of dioxane/water, ethanol/water, and methanol/water. The xsolvent is the molar fraction of solvent. (b) Fluorescence intensity data of (a) as a function of the effective dielectric constant of the solvent mixtures. Dielectric constants are from refs 34-36.

Figure 5. Fluorescence emission spectra (λexc ) 345 nm) of 1 in surfactant solutions of (a) SDS ([1] ) 10.0 µM) and (b) CTAB ([1] ) 3.3 µM).

formation of monomers can be explained on the basis of solubilization of 1 by the surfactant micelles. It is of interest to note that the intensity of the excimer band as a function of surfactant concentration shows a slight maximum well below the CMC. This phenomenon, however, is well-known and may be ascribed to a premicellar effect, that

is, the formation of undefined aggregates between the probe and surfactant molecules.46 This phenomenon is currently under investigation for our system. Determination of Critical Micellar Concentrations. Figure 6 shows the results of the CMC measurements of various types of surfactants. In these experiments, we employed two ionic

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Mohr et al.

Figure 6. Fluorescence intensity of 1 at λem ) 382 nm as a function of surfactant concentration. Surfactants employed were (a) SDS ([1] ) 10.0 µM), (b) CTAB ([1] ) 3.3 µM), (c) Triton X-100 ([1] ) 3.3 µM), (d) n-dodecyl-β-maltopyranoside ([1] ) 1.7 µM), (e) Brij-58 ([1] ) 1.7 µM), (f) Tween-20 ([1] ) 1.7 µM), (g) C12E4 ([1] ) 1.7 µM), and (h) C12E8 ([1] ) 1.7 µM).

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TABLE 1: Critical Micelle Concentrations (CMC) of Selected Surfactants

of II/IIII between the first (λmax ) 373 nm) and third (λmax ) 383 nm) vibronic bands of the monomer emission is usually plotted against the total surfactant concentration, giving rise to a decreasing sigmoidal curve of the Boltzmann type. However, a critical examination of the above-mentioned method20 has shown that the CMC can be obtained from two singular points, either from the inflection point of the sigmoid trace or the crossing point of the linear fits to the increasing and the horizontal part of the plot at high surfactant concentration. As mentioned above, this problem was extensively covered in the study of nonionic surfactants.20,53,58,62,63 For probe molecules which are incorporated in the micelles, the critical micelle concentration must be expected to be dependent on the concentration and the size of the probe molecules. Hence, the presence of 1 might affect the aggregation process and disturb the micellar aggregates. However, considering the excellent agreement between the CMCs determined in this study and the literature data (Table 1), this effect appears to be of minor importance with regard to the concentrations of 1 employed here. In order to further improve the reliability of our method, we are currently studying the micellization of a wider range of ionic and nonionic surfactants, with particular attention to the influence of other solutes, for example, salts. Results will be compared with those obtained by using conventional fluorescence probes such as DPH (1,6-diphenyl-1,3,5-hexatriene), ANS (8-anilinonaphthalene sulfonic acid magnesium salt), and pyrene.

CMC/mM surfactant

this work

literature values

ref

SDS CTAB Triton X-100 n-Dodecyl-βmaltopyranoside Brij-58 Tween-20 C12E4 C12E8

8.2 0.90 0.27 0.21

8.1-8.4 0.92-1.00 0.24-0.27 0.17

7, 9, 20, 47, 48 7, 9, 20 20, 50-53 18, 57

0.023 0.028 0.075 0.110

0.007-0.077 0.049 0.059-0.095 0.071-0.148

54, 60 18, 55 3, 20, 56, 59 3, 20, 58, 59

(SDS and CTAB) and four nonionic surfactants (Triton X-100, n-dodecyl-β-maltopyranoside, Brij-58, Tween-20, C12E4, and C12E8). These surfactants were selected because their literature CMC data are reliable.7,9,18,20,47-60 In order to determine the critical micelle concentrations, the fluorescence intensity of the most intensive peak at λmax ) 382 nm was plotted against the surfactant concentration. The concentration of 1 was adjusted according to the CMC of the surfactant in order to achieve a good signal-to-noise ratio, but care was taken that the concentration of 1 never exceeded a 7.5 molar percentage with respect to the CMC. As evident from Figure 6, all plots were adequately described by a sigmoidal function of the Boltzmann type. The critical micelle concentrations were determined at the inflection points of the fitted sigmoid traces. The evaluated numbers (at 22 ( 1 °C) are summarized in Table 1 together with literature data (at 25 °C). The data of Table 1 confirm the excellent agreement of the critical micelle concentrations determined in this study with the literature values. The sigmoidal shape of the fluorescence intensity/[surfactant] traces of Figure 6 indicates that well below the CMC, the emission spectrum of 1 corresponds to a “water-like” environment, showing no monomer emission of the pyrene fluorophore. When the surfactant concentration is increased, the appearance of the three peaks at λmax ) 382, 404, and 425 nm indicates a decrease in polarity and, thus, a more hydrophobic environment. Well above the CMC, the intensity of peak I remains roughly constant and independent of the surfactant concentration, indicating essentially complete incorporation of the probe molecules into the hydrophobic region of the micelles. However, Figure 6 shows that for Brij-58 and Tween-20, the fluorescence intensity at 382 nm increases with the surfactant concentration in a more gradual way than that found for the other surfactants. This behavior is particularly known for technical grade surfactants and is generally associated with the polydispersity, that is, purity, of the surfactant. In order to verify this aspect, we determined the CMCs of two pure (monodisperse) nonionic surfactants of the alkyl ether type, namely, C12E4 and C12E8. As evident from Figure 6g,h and Table 1, well-defined CMCs were obtained. Thus, the difference of the present and the literature CMCs for Brij-58 and Tween-80 (Table 1) is reasonably related to their polydispersity, notwithstanding that for some nonionic surfactants, the observed effect has also been explained either in terms of a premicellar effect or by the formation of small aggregates between the probe molecule and the surfactant.61 As mentioned above, the pyrene I/III ratio method of Kalyanasundaram and Thomas has become one of the most popular methods for the determination of critical micelle concentrations.9,20 In aqueous solution, the emission spectrum of pyrene shows a vibronic fine structure of five significant bands, which is quite sensitive to the local environment. In order to investigate critical micelle concentrations, the intensity ratio

Conclusions The fluorescence spectroscopic investigations on 1 presented here demonstrate that this compound can advantageously be employed as a probe molecule for the reliable determination of the CMC of surfactant compounds. Using 1 as a probe, two types of emission domains are of major importance, that is, the monomer emission with a three peak maxima at λmax ) 382, 404, and 425 nm and the excimer emission with a broad, redshifted band in the range 400-550 nm (λmax ) 465 ( 3 nm). Excimer emission arises in aqueous solution from direct excitation of ground-state dimeric aggregates of 1, associated due to hydrophobic interactions, whereas in micellar solution, the monomer emission originates from excitation of isolated molecules. In aqueous solution, it was therefore found that the fluorescence spectrum of 1 is sensitive to changes in the surfactant concentration. Hence, in going from pure water to micellar solution, a sigmoidal increase in the intensity at λmax ) 382 nm was used to determine the critical micelle concentration of a number of surfactants. The novel method described here represents a simple and sensitive approach for studying the micellization of surfactants. During the past decades, intensive effort has been made to characterize other types of assemblies such as vesicles, liposomes, microemulsions, polymers, and biological membranes. Further, polymer-surfactant interactions are of major practical importance for a wide range of applications, for example, hair conditioners, cosmetics, paints and inks, and pharmaceutical suspensions. For the study these systems, 1 may be a promising new probe molecule. Acknowledgment. We are grateful to Heinz Bandmann, Universita¨t Duisburg-Essen, for performing the NMR experiments and for helpful discussions. We thank two anonymous referees for helpful suggestions. Supporting Information Available: Synthetic procedures and spectral data for all compounds of Scheme 1. 1H, 13C, and

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